Recently, since it is easy to carry and use portable electronic devices, the use of the portable electronic devices is increasing day by day. These portable electronic devices such as portable terminals, digital cameras, or notebook computers may be required to have a waterproof function due to they are carried and used with the portability. However, sound holes are formed to emit sound at a portion where a speaker or microphone is installed, and accordingly water and dust are penetrated into an electronic device through the sound holes.
Thus, a waterproof sound-permitting sheet is provided in the sound holes to pass the sound but to block water or dust. For water resistance of the waterproof sound-permitting sheet, it is advantageous to reduce an average diameter of fine holes, and for sound-permittivity of the waterproof sound-permitting sheet, it is advantageous to enlarge the size of the fine holes. Therefore, it is important to maintain the average diameter of the fine holes as appropriate so as to satisfy two conditions such as the sound-permittivity and the waterproof.
As disclosed in Korean Patent Application Publication No. 10-2010-0041839 (published on Apr. 22, 2010), a conventional waterproof sound-permitting film includes a polytetrafluoroethylene porous film, in which the polytetrafluoroethylene porous film includes: a first porous layer; and a second porous layer stacked on and integrated with the first porous layer based on a settlement force acting between a matrix of polytetrafluoroethylene, surface density of the waterproof sound-permitting film is 1 to 20 g/m2, the first porous layer and the second porous layer are biaxially oriented, and a draw ratio of the first porous layer is equal to that of the second porous layer.
Such a waterproof sound-permitting film is configured to have a double layer structure formed of the first porous layer and the second porous layer, to thereby improve the waterproof performance. However, since the conventional waterproof sound-permitting film is formed of only a polytetrafluoroethylene porous film, fine holes of the porous film will increase gradually in size due to the pressure of the shock or sound externally applied due to the long use, and thus there is a problem that waterproof performance is reduced.